Diesel engine exhaust contains particulate matter, the emission of which is regulated for environmental and health reasons. This particulate matter generally constitutes a soluble organic fraction (“SOF”) and a remaining portion of hard carbon. The soluble organic fraction may be partially or wholly removed through oxidation in an oxidation catalyst; however, this typically results in a reduction of only about 20 percent of total particulate emissions. Thus, vehicles equipped with diesel engines may include diesel particulate filters for more completely removing the particulate matter from the exhaust stream, including the hard carbon portion. Conventional wall flow type diesel particulate filters may have particulate removal efficiencies of about 85 percent. However, diesel particulate filters, particularly those that have relatively high particulate filtration efficiency, are generally associated with high back pressures because of the restriction to flow through the filter. Further, with use, soot or other carbon-based particulate matter accumulates on the diesel particulate filters causing the buildup of additional undesirable back pressure in the exhaust systems. Engines that have large particulate mass emission rates may develop excessive back pressure levels in a relatively short period of time. High back pressures decrease engine efficiency and reduce engine performance. Therefore, it is desired to have diesel particulate filtration systems that minimize back pressure while capturing a high percentage of the particulate matter in the exhaust.
To prevent diesel particulate filters from becoming excessively loaded with particulate matter, it is necessary to regenerate the diesel particulate filters by burning off (i.e., oxidizing) the particulates that accumulate on the filters. It is known to those of skill in the art that one method by which particulate matter may be oxidized is to raise the temperature of the exhaust gas sufficiently to allow the excess oxygen in the exhaust gas to oxidize the particulate matter. Also well-known to those of skill in the art is that particulate matter may be oxidized at a lower temperature in the presence of sufficient amounts of nitrogen dioxide (NO2).
Diesel exhaust inherently contains nitrogen oxides (NOx), which consist primarily of nitric oxide (NO) and nitrogen dioxide (NO2). Typically, the NO2 inherently present in the exhaust stream is a relatively small percentage of total NOx, such as in the range of 5 to 20 percent but usually in the range of 5 to 10 percent. Although some regeneration of a diesel particulate filter occurs at such levels, it is insufficient to result in complete regeneration. The effectiveness of NO2 in regenerating a particulate filter depends in part on the ratio of NOx to particulate matter in the exhaust stream. This ratio varies across engine applications and engine model years, as both NOx and particulate matter are regulated species of diesel engine exhaust and the regulated limits vary by year. For example, for on-highway diesel engines, under the emissions regulations in place during the period from 1994 to 2002, the nominal emissions limits over the Federal Test Procedure (“FTP”) were 0.1 grams per brake horsepower hour for particulate matter and 4 to 5 grams per brake horsepower hour for NOx. This yields a NOx/particulate ratio of 40 to 50. During the period from 2003 to 2006, the nominal limits are 0.1 grams per brake horsepower hour for particulate matter and 2.5 grams per brake horsepower hour for NOx, yielding a NOx/particulate ratio of 25. During the period from 1991 to 1993, the nominal emissions limit for particulate matter was 0.25 grams per brake horsepower hour and the nominal emissions limit for NOx was 5 grams per brake horsepower hour over the FTP test cycle, giving a NOx/particulate ratio of 20. In the period from 1988 to 1990, the nominal particulate emissions limits were 0.6 grams per brake horsepower hour and the nominal NOx emissions limits were 6 to 7 grams per brake horsepower hour over the FTP test cycle, giving a NOx/particulate ratio of about 10. In summary, older engines tend to have lower NOx/particulate ratios (although the ratio does decrease for 2003 to 2006 engines relative to 1994 to 2002 engines). A lower NOx/particulate ratio means that there is potentially less NO2 in the exhaust stream per unit of particulate matter captured on the particulate filter, making regeneration of the particulate filter by NO2 more difficult. A higher NOx/particulate ratio means that there is more NO2 in the exhaust stream per unit of particulate matter, so that the particulate trapped on a particulate filter may more readily be regenerated by NO2.
To promote full regeneration, it is often necessary to increase the quantity of NO2 in the exhaust stream. This is particularly true where the NOx/particulate ratio is relatively small. One method to produce sufficient quantities of NO2 is to use an oxidation catalyst to oxidize a portion of the NO present in the exhaust stream to NO2. However, although nitrogen oxides have been a regulated constituent of diesel exhaust for some time, recent developments have suggested that emissions of NO2 should be regulated separately from overall NOx because of the adverse environmental and health effects of NO2. Therefore, it is desired that a diesel exhaust treatment system does not cause excessive increases in the amount of NO2 within the exhaust stream. One regulation proposed in California requires that the ratio of NO2 to NOx in the exhaust gas downstream from an exhaust treatment system be no more than 20 percentage points greater than the ratio of NO2 to NOx in the exhaust gas upstream from the exhaust treatment system. In other words, if the engine-out NOx mass flow rate is (NOx)eng, the engine-out NO2 mass flow rate is (NO2)eng, and the exhaust-treatment-system-out NO2 mass flow rate is (NO2)sys, then the ratio.
                    (                  N          ⁢                                          ⁢          O          ⁢                                          ⁢          2                )            ⁢      sys        -                  (                  N          ⁢                                          ⁢          O          ⁢                                          ⁢          2                )            ⁢      eng                  (              N        ⁢                                  ⁢        O        ⁢                                  ⁢        x            )        ⁢    eng  must be less than 0.20.
Applying an oxidation catalyst coating to a conventional diesel particulate filter may result in excessive NO2 emissions. Therefore, it is desired that a diesel exhaust treatment system capture a high percentage of the particulate matter in the exhaust while minimizing both the back pressure in the exhaust system and the emission of NO2 to the atmosphere.